Integrated circuits are used extensively in various existing technological systems including computer systems. As the use of computer systems continues to increase, so does the demand for improved computers such as smaller and faster computers. To keep pace with the demand for improved computers, the semiconductor industry has been producing devices operating at higher speeds.
Typically, a cost or consequence of a semiconductor device operating at a higher speed is a decrease in the breakdown voltage of the device. When the breakdown voltage of a transistor, for example, is exceeded, undesirable currents are generated that have to be compensated. One such undesirable current results from avalanche multiplication in the collector-base depletion region of a transistor.
Avalanche multiplication is a process in which energetic electrons collide with a lattice of the semiconductor and create additional free carriers. As the voltage applied to the collector is increased, the collector-base depletion region broadens. As a result of this broadening, an electric field intensity within the depletion region increases. Accordingly, the electric field accelerates the free carriers that are traversing this junction. As the electric field increases, the free carriers gain kinetic energy and momentum.
A few of the most energetic free carriers may have sufficient kinetic energy to create electron-hole pairs when colliding with atoms of the semiconductor lattice. The electron hole-pairs may contribute to collector and base currents. The newly liberated electron of the electron-hole pair may contribute to the collector-current by the electric field sweeping the liberated electron in the same direction as the free carrier, or incident electron, that liberated it. Thus, the collector-current may increase due to the collision event.
The newly generated hole from the electron-hole pair, however, may be swept by the electric field to the base and contribute to the base current. The hole may be directed from the collector-base depletion region toward the neutral base region and out of the base contact resulting in a negative base-current, or a base-current that is of the opposite polarity as a normal transistor base-current. The total base-current, therefore, may be reduced due to avalanche multiplication.
As the collector-emitter voltage continues to increase, the base-current continues to decrease and can actually become negative. The further the collector-emitter voltage is increased, the base-current may become more negative. The collector-emitter voltage at which the base-current crosses zero is defined as the collector-emitter breakdown voltage, BVCEO.
Consequently, transistors having a low BVCEO are not typically employed in high operating voltage environments but instead may be used within circuitry of devices having a low operating voltage to prevent avalanche multiplication and other undesirable results. The demand for faster operating computers, however, often requires circuitry with higher operating voltages and faster operating speeds. A trade-off, therefore, between the operating voltage and the operating speed of transistors in the circuitry of devices is continually made to produce improved devices.
Accordingly, what is needed in the art is circuitry, including transistors, that are capable of operating at a high speed and at a high operating voltage.